Avian Influenza

Cause

Transmission

Human infections with highly pathogenic avian influenza A(H5N1) virus
occur through bird-to-human, possibly environment-to-human and, very
rarely, limited, non-sustained human-to-human transmission. Direct contact
with infected poultry, or with surfaces and objects contaminated by their
droppings, is the main route of transmission to humans. Exposure risk is
considered highest when there is contact with infected avian faecal material
in the environment, especially during slaughter, de-feathering, butchering
and preparation of poultry for cooking. There is no evidence that properly
cooked poultry or poultry products can be a source of infection.

Nature of the disease

Patients usually present initially with symptoms of fever and influenza-like
illness (malaise, myalgia, cough, sore throat). Diarrhoea and other gastrointestinal
symptoms may occur. The disease progresses within days and many
patients develop clinically apparent pneumonia with radiographic infiltrates
of varying patterns. Sputum production is variable and sometimes bloody.
Multi-organ failure, sepsis-like syndromes and, uncommonly, encephalopathy
occur. The fatality rate among hospitalized patients with confirmed
H5N1 infection has been high (about 60%), most commonly as a result of
respiratory failure caused by progressive pneumonia and acute respiratory
distress syndrome. Fatal outcome had also been reported for H7N7 infection
in human. However, other avian influenza subtypes (e.g. H9N2) appear to
cause mild diseases.

Geographical distribution

Extensive outbreaks in poultry have occurred in parts of Africa, Asia, Europe
and the Middle East since 1997, but only sporadic human infections have
occurred to date. Continued exposure of humans to avian H5N1 viruses
increases the likelihood that the virus will acquire the necessary characteristics
for efficient and sustained human-to-human transmission through
either gradual genetic mutation or reassortment with a human influenza A
virus. Between November 2003 and July 2008, nearly 400 human cases of
laboratory-confirmed H5N1 infection were reported to WHO from 15 countries
in Africa, south-east and central Asia, Europe and the Middle East.

Risk for travellers

H5N1 avian influenza is primarily a disease of birds. The virus does not easily
cross the species barrier to infect humans. To date, no traveller is known to
have been infected.

Prophylaxis

Neuraminidase inhibitors (oseltamivir, zanamivir) are inhibitory for the
virus and have proven efficacy in vitro and in animal studies for prophylaxis
and treatment of H5N1 infection. Studies in hospitalized H5N1 patients,
although limited, suggest that early treatment with oseltamivir improves
survival. Late intervention with oseltamivir is also justified. Neuraminidase
inhibitors are recommended for post-exposure prophylaxis in certain
exposed individuals. At present WHO does not recommend pre-exposure
prophylaxis for travellers but advice may change depending on new findings.
Inactivated H5N1 vaccines for human use have been developed and
licensed in several countries but are not yet generally available; however,
this situation is expected to change. Some countries are stockpiling these
vaccines as a part of pandemic preparedness. Although the vaccines are
immunogenic, their effectiveness in preventing the H5N1 infection or
reducing disease severity is unknown.

Precautions

In affected countries, travellers should avoid contact with high-risk environments
such as live animal markets and poultry farms, any free-ranging or
caged poultry, or surfaces that might be contaminated by poultry droppings.
Travellers in affected countries should avoid contact with dead migratory
birds or wild birds showing signs of disease, and should avoid consumption
of undercooked eggs, poultry or poultry products. Hand hygiene with frequent
washing or use of alcohol rubs is recommended. If exposure to individuals
with suspected H5N1 illness or severe, unexplained respiratory illness
occurs, travellers should urgently consult health professionals. Travellers
should contact their local health providers or national health authorities for
supplementary information.